Wind tunnel with a closed section for aeroacoustic measurements with an anechoic coating

ABSTRACT

A wind tunnel with a closed section for aeroacoustic measurements with an anechoic coating including: a) a first cavity having a thickness D 1  in a range of 20≦D 1 ≦50 mm filled with a fibrous material with a flow resistance R in a range of 10≦R≦50 kRayl/m; b) a first microperforated panel having a thickness t 1  in a range of 0.25≦t 1 ≦0.75 mm and with a perforation percentage p 1  in a range of 15% ≦p 1 ≦30%; c) a second air cavity having a thickness D 2  in a range of 10≦D 2 −30 mm; d) a second microperforated panel having a thickness t 2  in a range of 0.25≦t 2 ≦0.75 mm and with a perforation percentage p 2  in a range of 15%≦p 2 ≦30%. The perforations of the panels can be circular or longitudinal slot-shaped having a diameter d or width w in a range of 0.2≦d, w≦0.5 mm.

FIELD OF THE INVENTION

The present invention relates to a wind tunnel with a closed sectionprovided with an anechoic coating for taking aeroacoustic measurementsand more particularly to a wind tunnel for conducting aerodynamic andacoustic tests of scale aircraft models.

BACKGROUND OF THE INVENTION

Wind tunnels are used to conduct aerodynamic and aeroacoustic tests ofscale models of various types of vehicles and particularly of aircraft.Aerodynamic tests traditionally use a closed section configurationbecause it is a mature technique in which the air flow impinging on themock-up is not really affected by the walls and because the associatedcorrections are very well known. However, the measurements aeroacousticare usually performed in an open section because reverberations in thewalls of the tunnel are thus avoided. This means that the aerodynamicand aeroacoustic tests are done separately, with the subsequentduplication of efforts and costs.

Even though various materials capable of absorbing sound are well knownin the prior art, no specific proposal, however, providing a coating ofa closed section of a wind tunnel with a high degree of acousticabsorption which allows easily taking the measurements which arerequired in aeroacoustic tests of, particularly, aircraft models, isknown.

The commercial solutions for the absorption of sound using porous andfibrous materials are not applicable for said coating due to the factthat the air jet circulating inside the tunnel at a high speed would endup carrying off said materials, with the subsequent loss of the acousticabsorption properties.

In relation to microperforated panels (MMPs) which, in theory can beconsidered applicable to said coating, several proposals which have beenused in several industrial sectors are known. The MPPs proposed by Maa(D. Y. Maa, (1997), “Potential of microperforated panel absorber” J.Acoust. Soc. Am., 104, 2861-2866) provide absorption of the sound byvisco-thermal losses in sub-millimetric perforations made on a panel andtherefore do not require the addition of fibrous materials. In order totune the absorption in the frequency band of interest, it is necessaryto have these MPPs in front of a rigid wall, leaving an air cavityhaving a certain thickness.

The use of MPPs for the absorption of the sound in various environmentsis well known in the art and has been the object of several patents.

U.S. Pat. No. 5,700,527 describes the use of microperforated glass asabsorbent materials in the construction of buildings. They are simpleglass MPPs having a thickness t in the range of 0.2≦t≦30 mm, withcircular perforations having a diameter d in the range of 0.1≦d≦2 mm andair cavities having a thickness D in the range of 20≦D≦500 mm.

ES 2 211 586 describes the use of MPPs for coatings in means oftransportation, such as land vehicles, trains, ships and airplanes, withpanels having a thickness t in the range of 0.2≦t≦5 mm, perforationshaving a diameter d in the range of 0.05≦d≦2 mm, and perforationpercentages p in the range of 0.2≦p≦4%. The air cavity in this case canbe filled with spongy material or wadding.

EP 1 382 031 describes the use of multilayer MPPs for absorbent coatingsengine exhaust systems or in turbines. The metal have thicknesses oft<0.2 mm, perforations having diameters of d<1 mm, and perforationpercentages of p<1%.

U.S. Pat. No. 6,675,551 describes low-cost thick MPPs for theirapplication as constructive elements. The panels can be made of wood,synthetic material or laminated gypsum, and can be combined with otherabsorbent materials, such as foams, mineral wools or acoustic fabrics.The panels can have thicknesses t in the range of 6≦t≦30 mm, withcircular perforations having diameters d≦2 mm, and perforationpercentages of p≦4%.

U.S. Pat. No. 6,617,002 describes MPPs using polymeric films. Sincethese films have a rigidity of less than 10⁷ dynes/cm, with a thicknessof t<0.38 mm, the model includes its elastic properties. Another noveltyof this patent is the use of conical perforations, with a major diameterof d₁<0.5 mm, and a minor diameter of d₂<0.15 mm.

As can be inferred from the foregoing, the known proposals are aimed atsolving specific acoustic insulation problems very different from thoseof an anechoic coating of a wind tunnel.

However, it is desirable to have such coating and the present inventionaims to solve this need.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a coating for a closedsection of a wind tunnel providing absorption of the sound in afrequency band from 500 Hz to 14 kHz which facilitates takingaeroacoustic measurements.

Another object of the present invention is to provide a coating for aclosed section of a wind tunnel which allows conducting aeroacoustictests on aircraft models.

These and other objects are achieved with an sound-absorbing coatingcomprising, from the wall of the tunnel, the following components: a) afirst cavity filled with a fibrous material; b) a first microperforatedpanel (MPP); c) a second air cavity; d) a second microperforated panel(MPP), with the following features: the first cavity has a thickness D₁comprised in the range of 20≦D₁≦50 mm and is filled with a fibrous orporous material having flow resistivity R comprised in the range of10≦R≦50 kRayl/m; the MPPs have thicknesses t_(1,2) comprised in therange of 0.25≦t_(1,2)≦0.75 mm and a perforation percentage p_(1,2)comprised in the range of 15% ≦p_(1,2)≦30%; the second air cavity has athickness D₂ comprised in the range of 10≦D₂≦30 mm.

In a preferred embodiment, the perforations of the MPPs arecircular-shaped with a diameter d comprised in the range of 0.2≦d≦0.5mm. Coatings for anechoic sections for wind tunnels with optimalabsorption capacities are thus achieved.

In another preferred embodiment, the perforations of the MPPs arelongitudinal slot-shaped perforations oriented in the direction of thewind flow in the tunnel with a width w comprised in the range of0.2≦w≦0.5 mm. Coatings for anechoic sections for wind tunnels with anoptimal absorption capacity/cost ratio are thus achieved.

Other features and advantages of the present invention will beunderstood from the following detailed description of an illustrativeembodiment of the object of the invention in relation to the attachedfigures.

DESCRIPTION OF THE DRAWINGS

FIG. 1 a is a cross-section schematic view of a wind tunnel with ananechoic coating according to the present invention and FIG. 1 b is adetail view of the structure of this coating.

FIG. 2 is a schematic detail and plan and cross-section view of an MMPwith slot-shaped perforations.

FIG. 3 shows absorption curves in normal incidence of the anechoiccoating of a wind tunnel according to the present invention as afunction of the flow resistivity of the fibrous material of the firstcavity.

FIG. 4 shows absorption curves in normal incidence of the anechoiccoating of a wind tunnel according to the present invention as afunction of the perforation percentage of the second MPP.

FIG. 5 shows the absorption curves in normal incidence of the anechoiccoating of a wind tunnel according to the present invention for severalcombinations of the thicknesses of the first and the second cavity.

FIG. 6 shows absorption curves in normal incidence of the anechoiccoating of a wind tunnel according to the present invention as afunction of the diameter of the perforations of the first MPP.

FIG. 7 shows an absorption curve in normal incidence of an anechoiccoating of a wind tunnel according to a specific embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

It is considered that a suitable anechoic coating of a closed section ofa wind tunnel in order to be able to conduct aeroacoustic tests must beable to provide an average absorption in normal incidence of 90% in afrequency band from 500 Hz to 14 kHz (almost 5 octaves).

According to a preferred embodiment of the present invention, andfollowing FIGS. 1 a and 1 b, a closed section of a wind tunnel can beobserved, the inner wall 5 of which is provided with a coating formed bythe following components:

A cavity 11 having a thickness D₁ comprised in the range of 20≦D₁≦50 mmfilled with a fibrous or porous material of flow resistivity R comprisedin the range of 10≦R≦50 kRayl/m.

A first MMP 13 with thickness t₁ comprised in the range of 0.25≦t₁≦0.75mm and a perforation percentage p₁ comprised in the range of 15%≦p₁≦30%.

A second air cavity 15 having a thickness D₂ comprised in the range of10≦D₂≦30 mm.

A second MMP 17 having a thickness t₂ comprised in the range of0.25≦t₂≦0.75 mm and a perforation percentage p₂ comprised in the rangeof 15% ≦p₂≦30%.

The suitability of the mentioned coating for complying with the demandsof the wind tunnel has been validated using an absorption modeldependent on the relevant parameters of its components obtained from theinput impedance thereof, which depends on the acoustic impedance of theMPP and of the acoustic impedance of the porous material. For example,the impedance of the MPP can be known from Maa's equations (D. Y. Maa,1997, “Potential of microperforated panel absorber” J. Acoust. Soc. Am.,104, 2861-2866), and the impedance of the porous layer can be obtainedfrom the Allard and Champoux model (J. F. Allard and Y. Champoux, 1992,“New empirical equations for sound propagation in rigid frame fibrousmaterials”, J. Acoust. Soc. Am., 91, 3346-3353).

FIG. 3 shows the ratio between the absorption coefficient a of thecoating and the value for the flow resistance R of the porous materialof the cavity 11 for the remaining fixed parameters. The curves 21, 22,23, 24, 25 are those corresponding, respectively, to the followingvalues for R: 10000, 21000, 30000, 40000 and 50000. As can be seen,maximum absorption is obtained for a porous material with flowresistance of 20 kRayl/m. This is a typical value for the resistivity ofrock wools, for example.

FIG. 4 shows the ratio between the absorption coefficient a of thecoating and the perforation percentage of the second MPP 17 for theremaining fixed parameters. The curves 31, 32, 33, 34, 35 are thosecorresponding, respectively, to the following values for perforationpercentage: 10%, 15%, 20%, 25% and 30%. As can be seen, the greater theperforation percentage, the greater the absorption. The ratio betweenthe absorption coefficient a of the coating and the perforationpercentage of the first MPP 13 is similar.

FIG. 5 shows the ratio between the absorption coefficient a of thecoating and the thicknesses D₁ and D₂ of the two cavities 11, 15 for theremaining fixed parameters. The curves 41, 42, 43 are thosecorresponding, respectively, to the following pairs of values for D₁ andD₂: 5, 1.9; 3, 2; 4, 1. As can be seen, it is necessary to suitablycombine both thicknesses D₁ and D₂ to obtain a high absorption curve.

The perforations of the first and second MPPs 13 and 17 can havedifferent shapes, particularly circular and longitudinal slot shapes. Inthe first case, the diameter d of the perforations is comprised in therange of 0.2≦d≦0.5 mm.

FIG. 6 shows the ratio between the absorption coefficient a of thecoating and the diameter of circular perforations having diameter d ofthe first MPP 13 for the remaining fixed parameters. The curves 51, 52,53, 54 and 55 are those corresponding, respectively, to the followingvalues for d: 0.3, 0.4, 0.5, 0.6 and 0.7. As can be seen, the smallersaid diameter, the greater the absorption. The ratio between theabsorption coefficient a of the coating and the diameter d of circularperforations of the second MPP 17 is similar. It has been experimentallyverified that an absorption effect similar to that of the circularperforations with longitudinal slots the width w of which (see FIG. 2)is equal or similar to the diameter of the circular perforations. Thelongitudinal direction of said slots must preferably coincide with thedirection of the wind flow in the tunnel. The cost of making these slotsis considerably less than the cost of making circular perforations.

A specific coating has been carried out for a section of 200 mm inlength of a wind tunnel having a rectangular section of 200×200 mm withtwo metal MPPs 13 and 17, having parameters of t_(1,2)=0.5 mm,p_(1,2)=23% and longitudinal slot-shaped perforations having a width ofw=0.23 mm made with a laser, a first cavity 11 having a thickness ofD₂=40 mm, filled with a rock wool having flow resistivity of R=28kRayl/m, and a second air cavity 15 having a thickness of D₁=20 mm, madewith a honeycomb structure. FIG. 7 shows the absorption curve in normalincidence of this coating. The absorption coefficient in the frequencyband between 500 Hz and 15 kHz is 0.88.

The modifications comprised within the scope defined by the followingclaims can be introduced in the preferred embodiments described above.

1-6. (canceled)
 7. A wind tunnel comprising: a closed section foraeroacoustic measurements, wherein its inner walls comprise asound-absorbing coating which, from the inner walls, comprises: a) afirst cavity having a thickness D₁ in a range of 20≦D₁≦50 mm filled witha fibrous material with a flow resistance R in a range of 10≦R≦50kRayl/m; b) a first microperforated panel having a thickness t₁ in arange of 0.25≦t₁≦0.75 mm and with a perforation percentage p₁ in a rangeof 15% ≦p₁≦30%; c) a second air cavity having a thickness D₂ in a rangeof 10≦D₂≦30 mm; d) a second microperforated panel having a thickness t₂in a range of 0.25≦t₂≦0.75 mm and with a perforation percentage p₂ in arange of 15% ≦p₂≦30%.
 8. The wind tunnel according to claim 7, whereinthe first and second microperforated panels have circular sectionperforations.
 9. The wind tunnel according to claim 8, wherein thediameter d of the perforations is in a range of 0.2≦d≦0.5 mm.
 10. Thewind tunnel according to claim 7, wherein the first and secondmicroperforated panels have longitudinal slot-shaped perforations thewidth w of which is in a range of 0.2≦w≦0.5 mm.
 11. The wind tunnelaccording to claim 10, wherein the longitudinal slots are made indirections parallel to a direction of the wind flow in the tunnel. 12.The wind tunnel according to claim 7, wherein the first and secondmicroperforated panels are metal panels and the filling material of thefirst cavity is rock wool.